1. Field of Invention
The present invention relates to a signal transmission structure. More particularly, the present invention relates a signal transmission structure and a circuit substrate thereof capable of reducing the impedance mismatch of the signal transmission path.
2. Description of Related Art
The common circuit substrates are composed of alternately stacked multiple patterned-circuit layers and multiple dielectric layers. For example, the patterned-circuit layers are formed on the copper foils through the photolithography process and etching process, and a part of the non-etched copper foils can serve as a reference plane of the circuit substrates, such as a ground plane or a power plane for example. The dielectric layers are disposed between the patterned-circuit layers or the reference planes to isolate the stacked patterned-circuit layers or the reference planes. In addition, the stacked patterned-circuit layers are electrically connected to each other through the Plating-Through Holes (PTH) or the conducting vias.
As to the connection of the circuit layers, the conducting via, for example, can be fabricated by the sequential steps: forming holes on the dielectric layer by multiple photolithography process, and filling the holes with conducting materials to electrically connect at least two patterned-circuit layers. Besides, the PTH can be formed, for example, by using mechanical drilling method to drill through the stacked circuit layers and the dielectric layers or through the stacked circuit layers and a single dielectric core. And then, an electroplating layer is formed on the inner wall of the through hole by an electroplating method to electrically connect at least two patterned-circuit layers.
Please refer to FIG. 1, which schematically illustrates a cross-sectional view of a conventional circuit substrate. Take a substrate having six patterned circuit layers for example. The circuit substrate 100 has a stacked layer 110, comprising a first circuit structure 120, a core layer 130, and a second circuit structure 140. Wherein, the two via landing pads 124, 126 of the first circuit structure 120 are electrically connected by a conducting via 125. A reference plane 128 is formed on a first dielectric layer 127 above the via landing pad 126, serving as the reference of the electric potential for the signal transmission in the first circuit structure. Besides, the two via landing pads 144,146 of the second circuit structure 140 are electrically connected by a conducting via 145. A reference plane 148 is formed on a second dielectric layer 147 above the via landing pad 146, serving as the reference of electric potential for the signal transmission in the second circuit structure. Furthermore, the core layer 130 comprises a plurality of through holes 132 conducting the first surface 130a and the second surface 130b of the core layer 130. And the inner wall of each through hole 132 is covered with a conducting wall 134 to electrically connect with the via landing pad 126 of first circuit structure 120 and with the via landing pad 146 of second circuit structure 140. The hollow part of the through hole 132 can be filled with a dielectric material 150.
In FIG. 1, the two via landing pads 126,146 are respectively disposed, for example, on the first surface 130a and second surface 130b of the core layer 130 made of the dielectric material. And the two via landing pads 126, 146 respectively cover two ends of the through hole 132. The via landing pads 126,146 are electrically connected by a conducting wall 134. Thus, the signals from a signal line of first circuit structure 120 can be transmitted sequentially through a connecting pad 122, the conducting via 123, the via landing pad 124 which is inside of an opening 128a formed by etching the reference plane 128, the conducting via 125, and the via landing pad 126. Then, after passing through the conducting wall 134 inside the through hole 132 of the core layer 130, the signals are transmitted through the via landing pad 146 the conducting via 145, the via landing pad 144 which is inside of the opening 148a formed by etching the reference plane 148, the conducting via 143, and the connecting pad 142. Finally, the signals are transmitted to the outside receiver connected with the connecting pad 142. The signal transmission structure described above can form a signal-transmitting path between two devices or two ends (not shown).
Noting that in the prior art, the transmitting lines in the signal-transmitting path used to electrically connect with two devices or two ends have a consistent width; that is, the electric impedance for each part of the signal-transmitting path must be kept identical such that the characteristic impedance of the signal-transmitting path is a constant while electronic signals are transmitted in the signal-transmitting path. Especially for the high-speed and high-frequency signal transmission, a well-designed signal-transmitting path between two devices or two ends is required to reduce the reflection caused by the impedance mismatch of the signal-transmitting path. In other words, the signal-transmitting quality is improved by reducing the insertion loss during signal transmission and enhancing the return loss. However as shown by the dash circles in FIG. 1, there are the overlapping areas between the via landing pad 124 and the adjacent reference plane 128 and between the via landing pad 146 and the adjacent reference plane 148 for the conventional substrates. In other words, the region where the reference plane 128 is projected on the first surface 130a overlaps the via landing pads 126; likewise the region where the reference plane 148 is projected on the second surface 130b overlaps the via landing pad 146. Thus, when the signals are transmitted, especially through the via landing pads 126 or 146, the deviation in impedance will be induced by the effect of parasitic capacitance between the reference planes 128 and 148. It also means that it aggravates the impedance mismatch of the signal transmission path.
Furthermore, when the differential signals in the signal transmission structure are transmitted through the conducting walls 134, the electromagnetic field coupling between the conducting wall 134 would affect the current circulation path and electric field, thus changing the characteristic impedance between the differential signals. Thus, when the differential signals are transmitted through the two through holes 132 and 136, the signals would reflect and deteriorate as a result of the inconsistent impedance caused by the two conducting walls. Furthermore it reduces the quality of the transmitting signals.